Mid-Atlantic faces relentless downpours that could trigger more flash flooding
Flash flooding will remain a concern as the storms push through the mid-Atlantic region.
Downpours triggered serious flash flooding across portions of the mid-Atlantic at midweek, including areas recently soaked by Isaias, and forecasters say repeated downpours and more incidents of flooding are likely to continue into the weekend across parts of the Southeast, central Appalachians and mid-Atlantic.
A wedge of dry air suppressed rain farther to the south on Friday and that dry wedge may hold on Saturday as a budding tropical system spins offshore.
Flash flood watches and warnings were in effect from southeastern Pennsylvania and central New Jersey to along the North Carolina/Virginia border Thursday morning. But, by the midday hours on Thursday, downpours blossomed in the New York City metropolitan area and spread to along the southern coast of New England as well.
Torrents of rain already led to numerous flash flooding incidents across parts of Virginia and Maryland during midweek. Flash flooding in northern Virginia caused a 50 foot by 100 foot sinkhole to open up in Manassas Park and led to water rescues early Wednesday morning, according to The Washington Post. One car was swept by floodwaters into a creek, and two other cars were “in danger of going into the sinkhole,” Capt. Frank Winston of the Manassas Park Police told The Washington Post.
Fairfax County Police also reported that roads were closed due to high water early Wednesday.
During Wednesday evening, nearly 3 inches of rain fell in one hour at Baltimore-Washington International Airport in Maryland.
On Wednesday afternoon, videos emerged showing roads that looked like rivers in Ellicott City, Maryland, a community located west of Baltimore that has been plagued by flooding in years past, including particularly serious episodes of flash flooding in 2016 and 2018. The historic town, which lies in both the Tiber and Patapsco river valleys, is notorious for flooding with documented incidents dating back into the 1800s.
Officials warned residents to seek higher ground in the historic district as a flash flood warning was issued. Parts of Main Street were closed due to the dangerous flooding, according to Howard County authorities.
Wednesday's storms ignited along the leading edge of a dry air mass sagging southeastward from the Great Lakes region and settling across the interior Northeast. The push of dry air sputtered and stalled from part of the Ohio Valley to the mid-Atlantic and southern New England coasts on Thursday.
Along this boundary, the air will be very moist and could continue to fuel torrential rainfall. Since the ground is saturated along much of this same swath that includes many of the major metro areas from Baltimore, Washington, D.C., to Philadelphia and in part of the New York City area, there is an enhanced risk of flash urban and small stream flooding. Motorists should expect substantial delays.
Storms may unleash too much rainfall in a short duration, but the wet weather also ushered in some heat relief. Places like Philadelphia and Washington, D.C., that have been enduring days of sweltering 90-degree Fahrenheit heat had high temperatures about 5 to 10 degrees lower on Thursday thanks to the persistent storms and cloud cover. Thermometers have been teetering near the 90-degree mark in New York City, also had a similar trend.
A stiff breeze from the ocean is likely along the mid-Atlantic coast into Saturday. Meanwhile, windy conditions are forecast for southeastern New England on Saturday.
Might a tropical storm develop in the backyard?
There is a high chance an area of disturbed weather evolves into a tropical depression or perhaps Tropical Storm Kyle while moving away from the mid-Atlantic coast this weekend.
Not only can this make for rough surf and slightly above normal tides, but it may also make the air especially humid from New Jersey to Virginia into Saturday. Surf may settle down by Sunday along the mid-Atlantic coast, but rough surf and increasing rip currents seem likely along the southeastern New England coast this weekend. Boaters beware...Offshore seas could get rough for small craft in the waters around Cape Cod, Massachusetts, and the islands.
Farther south, much of the Southeast and the lower part of the mid-Atlantic will be in the zone where there can be a shower or thunderstorm at any time through this weekend with the bulk of the activity during the afternoon and evening hours.
Where downpours repeat or a storm stalls over a location, the risk of flash flooding will increase. Some moisture could not only reach areas that have become dry in recent weeks, but also other areas that have been picking up precipitation on a daily basis.
Thanks in large part to Fay and Isaias, some locations of the mid-Atlantic have received twice their normal rainfall over the past six weeks.
In Philadelphia two single days during the period have accounted for most of the rainfall. Fay unleashed 4.15 inches of rain on July 10, and Isaias dumped 4.16 inches of rain on Aug. 4. Total rainfall from July 1 through Aug. 11 was 11.17 inches, compared to a normal rainfall of 5.66 inches.
Meanwhile, in Atlanta, which dodged both tropical systems, rainfall since July 1 has only been about 45 percent of the average of 6.64 inches through Aug. 11. Boston also managed to avoid direct rainfall from both tropical systems with less than 2 inches of rain to show for the same approximate six-week period.
Even though some areas that have been dry recently will pick up some needed rainfall, not everywhere will get drenched by the upcoming pattern. Some of the neediest areas may be bypassed by what would be beneficial moisture. Abnormally dry to moderate drought conditions are affecting areas from central Pennsylvania up through much of New England, according to the U.S. Drought Monitor.
As the weekend progresses, moisture from the Southern states will percolate northward and moisture from the Plains will advance eastward. The leading edge of this shower and thunderstorm activity will overspread areas west of the Appalachians later Saturday and Saturday night. It is possible that downpours return to dry areas of Pennsylvania and New York state that will miss out on rainfall during the second half of this week.
"This is the type of pattern, especially in areas from Maryland and West Virginia on south, where there will be flooding downpours, but nearly impossible to say where the downpours will be from one day to the next and which areas will end up with the most rain over a five- to seven-day period," AccuWeather Senior Meteorologist Dan Kottlowski said.
Through this weekend, the areas most likely to be wet most often will be in the Southeastern quarter of the nation, and areas most likely to be dry most often will be from eastern upstate New York to central and northern New England.
During early next week, wet and unsettled conditions with showers and thunderstorms are likely to sweep across much of the Northeast, including areas that manage to stay dry much of the time through Sunday.
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